Basically, the said curves are referring to constant current motor operation.In constant current mode, there isn't a thing like maximum voltage rating (or more exactly, there is of course an insulation voltage strength, but it's surly much higher than 24 V).
In constant current mode, applied driver voltage is an additional parameter that influences the e.m.f. related torque reduction at higher speeds. A higher voltage means larger torque at high speed. In contrast, you get the worst torque versus speed characteristic in constant voltage mode, where the motor current is only set by the windings resistance.
In so far the question is rather referring to basic knowledge of step motor operation than datasheet details.